In the information age, the demand for data networks of higher and higher data capacities, at lower and lower costs is constantly increasing. This demand is fueled by many different factors, such as the tremendous growth of the Internet and the World Wide Web. The increasing numbers of on-line users of the Internet and the World Wide Web have greatly increased the demand for bandwidth. For example, Internet video clips require a large amount of data transfer bandwidth.
Optical fiber transmission has played a key role in increasing the bandwidth of telecommunications networks. Optical fiber offers much higher bandwidths than copper cable and is less susceptible to various types of electromagnetic interferences and other undesirable effects. As a result, optical fiber is the preferred medium for transmission of data at high data rates and over long distances.
In optical fiber communication systems, data is transmitted as light energy over optical fibers. The data is modulated on an optical light beam with an optical modulator. Optical modulators modulate the amplitude or the phase of the optical light beam. Direct optical modulators modulate the optical wave as it is generated at the source. External optical modulators modulate the- optical wave after it has been generated by an optical source.
One type of external modulator is an electro-optic interferometric modulator, such as a Mach-Zehnder interferometric (MZI) modulator, that is formed on a X-cut or Z-cut lithium niobate substrate. A MZI modulator is a dual waveguide device that is well known in the art. In operation, an electromagnetic signal, such as a RF or microwave signal, interacts with an optical signal in one of the waveguides over a predetermined distance that is known as the interaction distance. The RF signal propagates in a coplanar waveguide (CPW) mode.
Typical high-speed electro-optical external modulators use a traveling-wave electrode structure to apply the RF signal. Such modulators have a RF transmission line in the vicinity of the optical waveguide. The RF signal and the optical signal co-propagate over an interaction distance, thereby acquiring the required optical modulation. The bandwidth of such structures is limited by a phenomenon known as xe2x80x9cwalk off,xe2x80x9d which occurs when an electrical signal and an optical signal propagate with different velocities or group velocities.
A number of solutions have been suggested to limit xe2x80x9cwalk offxe2x80x9d or to match the velocity of the optical signal to the velocity of the RF signal. One method of velocity matching the RF signal to the optical signal is to include a buffer layer on the top surface of the substrate that increases the propagation velocity of the RF signal to a velocity that is closer to the propagation velocity of the optical signal. Another method of reducing velocity mismatch between the RF signal and the optical signal is to decrease the interaction distance. Decreasing the interaction distance, however, requires an increase in the electric field that is required to obtain a suitable phase shift in the optical signal.
A method of reducing velocity mismatch between the microwave modulation signal and the optical signal propagating in the waveguide includes providing a buffer layer that has approximately the same effective dielectric constant as the optical waveguide and also introducing electrically floating electrodes between RF electrodes and the substrate to maximize the electric field across the waveguide.
However, such a structure may induce undesired longitudinal current in the ground electrodes that are electro-magnetically coupled to the electrically floating electrodes. This undesired longitudinal current can negatively impact the performance of the modulator. For example, the undesired longitudinal current can result in coupled modes being created in the ground electrodes and in the electrically floating electrodes. The undesired longitudinal current can also result in conversion of the CPW mode to higher order modes in the ground electrodes and in the electrically floating electrodes. This modal coupling and modal conversion can lead to high frequency loss in the substrate, which can degrade modulator performance at high frequencies.
An electro-optic modulator according to the present invention uses an improved floating electrode mechanism for extending the electro-optic bandwidth of the optical device. An electro-optic modulator according to the present invention has relatively high bandwidth and does not experience high frequency loss that occurs in prior art modulators having known floating electrode structures.
An electro-optic modulator according to the present invention includes a plurality of electrodes that are segmented and coupled to ground. These electrodes substantially prevent the formation and propagation of high-order modes in the plurality of electrically grounded electrode segments and the plurality of electrically floating electrode segments. This substantially suppresses modal coupling to the substrate, thereby reducing the insertion loss in the optical waveguide and extending the electro-optic bandwidth of the device.
Accordingly, in one aspect the present invention is embodied in an electro-optic device, such as a Mach-Zehnder interferometric modulator, that includes a lithium niobate substrate having an optical waveguide that is formed in an upper surface of the substrate. In one embodiment of the invention, the lithium niobate substrate is cut perpendicular to the X-axis (X-cut lithium niobate). In another embodiment of the invention, the lithium niobate substrate is cut perpendicular to the Z-axis (lithium niobate).
The electro-optic device also includes a plurality of electrically floating electrode segments that are positioned on the substrate. The plurality of electrically floating electrode segments are adapted to intensify an electric field in the optical waveguide.
The electro-optic device also includes a plurality of electrically grounded electrode segments that are positioned on the substrate. The electrically grounded electrode segments substantially prohibit modal conversion and propagation of high order modes in the plurality of electrically grounded electrode segments and the plurality of electrically floating electrode segments by substantially interrupting the continuity of induced electrical current in the plurality of electrically grounded electrode segments. This substantially reduces high frequency loss in the substrate.
In one embodiment, each of the plurality of electrically grounded electrode segments is separated from an adjacent one of the plurality of electrically grounded electrode segments by a predetermined distance. In one embodiment, the predetermined distance is chosen so as to substantially suppress modal coupling to the substrate and propagation of higher order modes including one or more substrate modes in the plurality of electrically grounded electrode segments and the plurality of electrically floating electrode segments. In one embodiment, the predetermined distance is chosen so as increase or to maximize suppression of modal coupling to the substrate.
A buffer layer that comprises a dielectric material is formed directly on the upper surface of the lithium niobate substrate. In one embodiment, the buffer layer includes BCB dielectric material. In another embodiment, the buffer layer includes a SiO2 dielectric material. In another embodiment, the buffer layer includes a TF4 dielectric material. In another embodiment, the buffer layer includes a semiconductor material. In one embodiment, the buffer layer has a thickness that is less than ten microns.
The electro-optic device also includes a driving electrode that is formed on the buffer layer. The driving electrode is adapted to receive an RF signal that induces an electric field in the optical waveguide.
In another aspect, the present invention is embodied in a method for suppressing modal coupling to a substrate of an electro-optic device. The method includes inducing an electric field in an optical waveguide by applying an RF signal to a driving electrode. The method also includes intensifying the electric field in the optical waveguide by positioning a plurality of electrically floating electrode segments and electrically grounded electrode segments proximate to the optical waveguide.
The method further includes interrupting a continuity of induced electrical current in the plurality of electrically grounded electrode segments. This substantially prohibits modal conversion and propagation of high order modes in the plurality of electrically grounded electrode segments and the plurality of electrically floating electrode segments. In one embodiment, the interrupting the continuity of induced electrical current in the plurality of electrically grounded electrode segments includes positioning the plurality of electrically grounded electrode segments proximate to the plurality of electrically floating electrode segments. By interrupting the continuity of induced electrical current in the plurality of electrically grounded electrode segments, the method substantially suppresses modal coupling to the substrate, which reduces insertion loss in the electro-optic device.
In one embodiment, each of the plurality of electrically grounded electrode segments is separated from an adjacent one of the plurality of electrically grounded electrode segments by a predetermined distance. In one embodiment, the predetermined distance is chosen so as to substantially prohibit modal conversion and propagation of higher order modes including one or more substrate modes in the plurality of electrically grounded electrode segments. In one embodiment, the predetermined distance is chosen so as minimize high frequency loss in the substrate.
In one embodiment, the method further includes positioning at least one of the plurality of electrically floating electrode segments a distance from an adjacent one of the plurality of electrically grounded electrode segments so as to substantially modal conversion and propagation of high order modes including one or more substrate modes in the plurality of electrically grounded electrode segments and the plurality of electrically floating electrode segments. In one embodiment, the method further includes positioning at least one of the plurality of electrically grounded electrode segments a distance from an adjacent one of the plurality of electrically grounded electrode segments so as to increase or to maximize suppression of modal coupling to the substrate.
In one aspect, the present invention is embodied in an electro-optic modulator such as a Mach-Zehnder optical modulator. The modulator includes a substrate. A first and a second optical waveguide are formed in an upper surface of the substrate. The modulator also includes a plurality of electrically floating electrode segments that are positioned on the substrate. The plurality of electrically floating electrode segments intensifies the electric field in the first and the second optical waveguides.
The modulator further includes a plurality of electrically grounded electrode segments that are positioned on the substrate. The plurality of electrically grounded electrode segments substantially prohibits modal conversion and propagation of high order modes in the plurality of electrically grounded electrode segments and the plurality of electrically floating electrode segments, which suppresses modal coupling to the substrate. In one embodiment, each of the plurality of electrically grounded electrode segments is separated from an adjacent one of the plurality of electrically grounded electrode segments by a predetermined distance. The predetermined distance is chosen so as to substantially prohibit modal conversion and propagation of higher order modes including one or more substrate modes in the plurality of electrically grounded electrode segments and the plurality of electrically floating electrode segments. In one embodiment, the predetermined distance is chosen so as to increase or to maximize suppression of modal coupling to the substrate.
A buffer layer is formed on top of the substrate, the plurality of electrically floating electrode segments, and the plurality of electrically grounded electrode segments. The buffer layer can be a dielectric material, such as a BCB dielectric material, a TF4 or a silicon dioxide dielectric material. The buffer layer can also be a semiconductor material.
The modulator includes a driving electrode that is formed on the buffer layer. The driving electrode is adapted to receive a microwave or a radio-frequency (RF) signal from an RF input. The RF signal induces an electric field in the first and the second optical waveguides. In one embodiment, the modulator also includes an optical source for providing the optical signal to the first and the second optical waveguides.
In one aspect, the invention is embodied in a method for modulating an optical signal. The method includes inducing an electric field in a first and a second optical waveguide by applying an RF signal to a driving electrode. The method further includes intensifying the electric field in the first and the second optical waveguides by positioning a plurality of electrically floating electrode segments and a plurality of electrically grounded electrode segments proximate to the first and the second optical waveguides.
The method also includes interrupting a continuity of an induced electrical current in the plurality of electrically grounded electrode segments. This substantially prohibits modal conversion and propagation of high order modes in the plurality of electrically grounded electrode segments and the plurality of electrically floating electrode segments. In one embodiment, the interrupting of the continuity of the induced electrical current in the plurality of electrically grounded electrode segments substantially suppresses modal coupling to the substrate, thereby reducing insertion loss in the electro-optic device.
In one embodiment, the interrupting of the continuity of the induced electrical current in the plurality of electrically grounded electrode segments includes positioning the plurality of electrically grounded electrode segments proximate to the plurality of electrically floating electrode segments. In one embodiment, each of the plurality of electrically grounded electrode segments is separated from an adjacent one of the plurality of electrically grounded electrode segments by a predetermined distance. The predetermined distance is chosen so as to substantially prohibit modal conversion and propagation of higher order modes including one or more substrate modes in the plurality of electrically grounded electrode segments and the plurality of electrically floating electrode segments. In one embodiment, the predetermined distance is chosen so as increase or to maximize suppression of modal coupling to the substrate.
In one embodiment, the method further includes positioning at least one of the plurality of electrically floating electrode segments a distance from an adjacent one of the plurality of electrically grounded electrode segments so as to prohibit modal conversion and propagation of high order modes in the plurality of electrically grounded electrode segments and the plurality of electrically grounded electrode segments.